1. Field of the Invention
This invention relates generally to magnetic recording disk drives, including disk drives with bit-patterned-media (BPM) where each data bit is stored in a magnetically isolated data island, and more particularly to a disk drive that can recover written data from data sectors that have hard bit errors, such as defective data islands in BPM.
2. Description of the Related Art
Magnetic recording hard disk drives with bit-patterned-media (BPM) have been proposed to increase the data density. In BPM, the magnetic material on the disk is patterned into small isolated data islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous-media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. The data islands in the tracks are grouped into data sectors.
The writing of data to the data islands in the data sectors also includes the writing of parity bits or error correction bits. The use of an error correction code (ECC) in disk drives is well-known and involves writing parity bits or error correction bits at the ends of data sectors or data tracks, where the parity bits or error correction bits are computed from the data to be written, using an algorithm, like one of the well known ECC algorithms. When the data is read back from a sector the ECC detects errors and uses the error correction bits to correct the errors. Examples of error correction codes used in magnetic recording disk drives include block codes such as Reed-Solomon (RS) codes and convolutional codes such as low-density parity-check (LDPC) codes.
The ECC can successively decode the written data provided the number of incorrect bits falls below some threshold. However, it is well known that the allowed number of bit errors increases if the ECC is provided with pointers to unreliable bits or suspected bad bits. In BPM, defective data islands are a source of unreliable bits. In perpendicular magnetic recording (PMR) and heat assisted magnetic recording (HAMR) on conventional continuous-media, the unreliable bits may arise from defects in the continuous magnetic recording layer, such as asperities or local grain irregularities. In BPM combined with HAMR defective data islands are also a source of unreliable bits. These unreliable sections of media can lead to “hard errors”, i.e., errors that cannot be corrected by rereading because the media is improperly magnetized. The hard errors can result in data sector failures, i.e., the inability to correctly read back the data from the sector. Identifying the unreliable bits can allow the sector to be decoded properly. One approach to identifying defective data islands is to map the entire disk, typically during manufacturing or disk drive idle time, and catalogue the locations of the defective islands. However, tracking defective data islands in this manner is time and memory-space consuming and is ineffective if defects get created during operation.
What is needed is a disk drive that is able to determine the location of defective bits after a sector failure so that the ECC can decode the data written in the failed sector.